This invention relates to a refractory composition employing basic aggregates consisting primarily of magnesium and calcium oxides, phenolic resin and an acid compound having a pKa less than 9.5 at 25.degree. C.
Refractory aggregates are mixed with the resin and pressed or molded into the desired shape or structure, and the resin polymerizes to form a refractory, which may be subjected to further heat treatment to prepare the final refractory item. During the heating and processing cycles, the resin must maintain the refractory materials in the desired shape until the aggregates unite to form the final refractory. During this cycle, the temperatures may be such that the resin is destroyed and removed from the final refractory.
Materials such as tar and pitch have been used as binders for refractory aggregates but these materials were difficult to work with and could not be used for molding complex or intricate parts. Phenolic resins have generally replaced tars, pitches etc. and have provided processes that can be conducted at ambient temperatures and have reduced the environmental problems associated with refractory production. The use of phenolic resins has enabled the mass production of complex structured refractory materials.
It is known that a refractory composition made of basic aggregate and a phenolic resin colors to a reddish violet hue. When molded and fired, such a refractory material does not provide refractories which have satisfactory flexural strength. The coloring of reddish violet and the low flexural strength of refractories are pronounced when the refractory material is produced by wet process in which the mixing of refractory materials is accomplished in the form of water slurry. The flexural strength of refractories may be improved if phenolic resin binder is added in large quantities during the production of the refractory material. This leads to the cost increase of the refractory material, and excessive phenolic resin in the refractories generates a large quantity of hydrogen gas on decomposition which takes place when the refractories come into contact with molten metal. This hydrogen enters the molten metal and stays in the free form in cast iron, causing cold brittle fracture of cast iron. Phenolic resin is used as a binder for refractory materials on account of its merits; but phenolic resin has the above-mentioned disadvantages.
In order to overcome these disadvantages, the present inventors carried out a series of investigations which have led to the findings that a refractory material containing a compound having a pKa lower than 9.5 at 25.degree. C. and a phenolic resin in a specific ratio is free of the above-mentioned disadvantages.
pKa is the dissociation constant of acids. In the case of acids that undergo dissociation in multiple stages, pKa is the dissociation constant of the first stage.
The invention, disclosed herein produces a refractory that does not color to a reddish violet hue and provides refractions having improved flexural strength.